Avarage Comment rating in wordpress

<?php

/* Avarage Comment rating  */

function average_rating() {
    global $wpdb;
    $post_id = get_the_ID();
    $ratings = $wpdb->get_results("

        SELECT $wpdb->commentmeta.meta_value
        FROM $wpdb->commentmeta
        INNER JOIN $wpdb->comments on $wpdb->comments.comment_id=$wpdb->commentmeta.comment_id
        WHERE $wpdb->commentmeta.meta_key='rating'
        AND $wpdb->comments.comment_post_id=$post_id
        AND $wpdb->comments.comment_approved =1

        ");
    $counter = 0;
    $average_rating = 0;   
    if ($ratings) {
        foreach ($ratings as $rating) {
            $average_rating = $average_rating + $rating->meta_value;
            $counter++;
        }
        //round the average to the nearast 1/2 point
        return (round(($average_rating/$counter)*2,0)/2); 
    } else {
        //no ratings
        return '0';
    }
}